LW 20 ribs
Re: LW 20 ribs
Interesting question, but I'm afraid I don't have much to offer. My 1100 LT20 Magnum, Serial #M86****U was made in March 1978, and came with a 28" Full with a flat rib. I bought a 2-3/4" 26" Skeet barrel the same day I bought it, and it has a very slight "break" angle on the rib. The 28" fixed Full 2-3/4" barrel I bought later also has the slight break. No date codes on the latter two barrels, but both were pre-RemChoke so figure pre-1987. All shoot right to POA, I do know that.
What could have happened... did.
Re: LW 20 ribs
Remington has played around with rib and barrel designs. If you look at the early 1100 barrels the "vents" in the vent rib didn't start until you got a ways down the barrel; a long solid portion first. Have you gotten around to measuring lengths yet? Most Remington barrels measure different than the exact advertised length. Probably why they shoot better. 
What could have happened... did.